1. Field of the Invention
The present invention relates to preventing saturation of transistors, with application to amplifier distortion reduction.
2. State of the Art
In the communications field, communications signals are required to be produced with low distortion. Particularly in wireless communications, low distortion is required in order to avoid adjacent channel interference (sometimes referred to as xe2x80x9csplatterxe2x80x9d). A principle cause of distortion in wireless communications is saturation of a final amplifier stage, resulting in signal clipping.
U.S. Pat. No. 5,021,753 describes a linear amplifier including an arrangement for avoiding saturation of the amplifier. Conventional quadrature modulation is employed. In this arrangement an error signal is derived by comparing a desired signal component (I or Q) to a corresponding measured signal component obtained using a Cartesian feedback circuit. When the error exceeds a threshold, a pulse is generated. The frequency of occurrence of such pulses over time is monitored by a processor. The processor adjusts a scale factor within the circuit such that the frequency with which the error threshold is exceeded remains within a desired range.
The use of Cartesian feedback to control amplifier saturation introduces significant circuit complexity, particularly if the effects of feedback delay are to be compensated for. Moreover, such a technique is not readily applicable to an amplifier following a polar architecture such as that described in High Efficiency Modulating RF Amplifier, U.S. patent application Ser. No. 09/637,269, filed Aug. 10, 2000, incorporated herein by reference.
Accordingly, techniques are needed that avoid Cartesian feedback and that may be used with amplifier architectures other than the conventional quadrature modulator architecture. More generally, improved techniques are needed for avoiding transistor saturation.
The present invention, generally speaking, provides methods and apparatus for avoiding saturation of a transistor, with particular application to communications signal amplifiers. In accordance with one aspect of the invention, feedback circuitry is provided to control a transistor in accordance with a control signal. The combination of current monitor and a threshold comparator detects when the base current of the transistor exceeds a threshold, indicative of the onset of saturation. Feedback control is then modified to prevent saturation.